Thailand's king, the world's longest serving monarch, has been hospitalized on advice from his doctors. The hospitalization of the revered monarch is causing concern among the Thai public.

A statement from the Royal Palace said King Bhumipol Adulyadej was admitted to the hospital Saturday, suffering from fever and fatigue.

The 81-year-old king has presided over Thailand for more than 60 years and is deeply revered by Thais.

The palace says the king is receiving treatment and antibiotics on the advice of his doctors. He had visited the hospital earlier in the week.

Professor Thongthong Chandrangsu, secretary general of the Thai Education Council, says the king's long reign means he has a special place in Thai society.

"After being on the throne for over 63 years he is a supreme leader of this society," he said. "His work and his actions express his sincere concern for the benefit and the wealth of the Thai people. So we appreciate his leadership and we wish he will be our king forever."

Prime Minister Abhisit Vejjajiva, who is traveling to the United States this week, told reporters prior to his departure the doctors had assured him the king's health was not a cause for concern.

Crowds flocked to the Siriraj hospital Sunday as news spread of the king's illness. Professor Thongthong said people had gathered to wish the king a quick recovery.

"Thousands of people gathered around the Siriraj [hospital] area with the same feeling in our hearts for to humbly ask His Majesty the king to return to the healthy situation as soon as possible," he said.

The issue of the king's health is highly sensitive in Thailand as he has faced health concerns over recent years. In 2007, he suffered symptoms associated with a minor stroke. He has also received treatment for a heart condition.

Last December, King Bhumipol was unable to deliver his annual birthday speech to the nation due to health reasons.

The king, while being viewed as above politics, has played a stabilizing role over the years in Thailand's often fractious political climate.

On several occasions he has used his significant influence to ease tensions or call on political adversaries to end their dispute.